16178
2008, 112, 16178–16181 Published on Web 09/25/2008
Rubrene Heteroepitaxial Nanostructures With Unique Orientation Marcello Campione* Department of Materials Science and CNISM, UniVersity of Milano Bicocca, I-20125 Milan, Italy ReceiVed: August 1, 2008; ReVised Manuscript ReceiVed: September 9, 2008
The development of a thin film technology based on the outstanding organic semiconductor rubrene is seriously prevented by its tendency to solidify in an amorphous state when deposited on foreign substrates. Here, a crystalline thin film of rubrene was successfully grown by vapor deposition on the surface of a tetracene single crystal. Atomic resolution imaging performed with scanning force microscopy revealed the high crystallinity of the film and its unique orientation achieved through a line-on-line epitaxial relation. An increasing scientific and technological interest is being devoted to organic semiconductors for their promising applications in large-scale production devices such as organic field effect transistors (OFETs), light-emitting diodes, photovoltaic cells, and sensors.1 Within the wide group of materials tested as active layer in (opto)electronic devices, a position of prime interest is occupied by rubrene (C42H28, 5,6,11,12-tetraphenyltetracene). Thanks to a favorable overlapping of π-orbitals of adjacent molecules, this material in its crystalline phase displays the highest hole mobility measured in organic semiconductors, reaching the value of 20 cm2/Vs.2 Due to this outstanding mobility, crystalline rubrene is one of the most interesting organic semiconductor for the development of OFETs,3 so that many recent studies are devoted to the processing of this material in its single crystalline phase.4 Nonetheless, despite its potentiality in the development of highperformance and low-cost devices, a major limit is being encountered for its integration in nanotechnology: the difficulty to process rubrene from the vapor phase for growing crystalline thin films. Indeed, when deposited by sublimation in a vacuum, it solidifies forming an amorphous film on a large variety of substrates.5 It is obvious that charge mobility is extremely limited when conjugated molecules are arranged in a disordered phase. Thus, the possibility to grow a crystalline thin film phase of rubrene is of crucial importance for the development of a nanotechnology based on its distinguishing performances. Recent successful attempts were reported: hot-wall deposition was employed for growing crystalline rubrene films on silica and Au(111) substrates;6 a passivation layer of self-assembled octadecyltrimethylsilane on silica was used together with an increment of the substrate temperature during deposition for inducing the crystallization of platelets of rubrene;7 finally, covering the substrate with a buffer layer of pentacene was demonstrated to enhance to a substantial extent the crystallinity of deposited rubrene.8 It is worth noting that the inadequacy of molecular beam deposition claimed in pioneering studies on the growth of rubrene thin films6 was progressively denied by * Corresponding Author. Address: Department of Materials Science & CNISM, University of Milano Bicocca, I-20125 Milan, Italy. E-mail:
[email protected], Tel: +39 02 64485012, Fax: +39 02 64485400.
10.1021/jp806877e CCC: $40.75
subsequent experiments until, in a definitive way, after the demonstration of the homoepitaxial growth of rubrene.9 In this letter, rubrene is deposited by sublimation from a highly purified source on the (001) surface of a single crystal of tetracene without any postgrowth treatment; a morphological analysis performed by atomic-force microscopy (AFM) from micrometer- to atomic-scales reveals the presence of a highly crystalline orthorhombic phase displaying a unique orientation in accordance to a line-on-line epitaxial relation.10 This represents full evidence of the possibility to grow crystalline and highly oriented nanostructures of rubrene on a foreign substrate, ensuring the full exploitation of the well-known performances of this material when integrated at the nanoscale. Furthermore, tetracene itself, used here as a single crystal substrate, has a manifold scientific interest. Besides the established effectiveness of organic substrates in tuning the molecular orientation of organic overlayers,11 the low symmetry of the crystalline phase of tetracene makes it a good candidate as a substrate for growing highly oriented organic nanostructures, and it was recently employed for growing uniaxially oriented organic-organic nanostructures of R-quaterthiophene.12 Tetracene single crystals were also successfully used for fabricating organic field-effect transistors (OFETs)13 and solar-cells,14 whereas its thin film phase found applications in OFETs,15 light-emitting OFETs,16 ambipolar OFETs,17 and solar cells.18 As exemplified by the aforementioned group of organic devices, the combination of different organic semiconductors in multilayer structures often improves device operation; however, the control of the molecular arrangement at the interface influences drastically the ultimate physical properties of the system. For the rubrene/tetracene heterostructure, the interface structure is here fully resolved, providing the key mechanism responsible for this efficient orientation effect. The understanding of this mechanism will offer the strategy for substrate selection and/or nanoscale modification for growing rubrene on other substrates of technological relevance. The tetracene substrates employed for this study were grown by physical vapor transport,19 obtaining thin (001)-oriented flakes which spontaneously adhere to a glass plate. This plate is then placed in the growth chamber and kept at 100 °C during rubrene deposition. 2008 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16179
Figure 1. AFM images (20 × 20 µm2) showing the surface morphology evolution with coverage of the same area of a rubrene thin film deposited on tetracene(0 0 -1). The film nominal thickness is indicated on the top-right of each image. The crystallographic orientation of the substrate surface is indicated in the top-left image by black arrows.
Figure 1 reports a sequence of AFM images collected on the same area of the sample after subsequent depositions of rubrene, until a nominal film thickness of 11 nm is reached. For a thickness of 2 nm, fractal islands extending over an area of about 100 µm2 are formed. Islands appear with a quite uniform contrast, indicating a narrow distribution of their heights. For a thickness as low as 1 nm, islands are observed to be formed by 4-5 layers, each with 1.5 ( 0.2 nm height (see Supporting Information, Figure S2). Such a layer spacing is consistent with (100)-oriented crystalline domains of the orthorhombic phase of rubrene (d(200) ) 1.343 nm).20 As coverage increases, islands are observed to grow both in height and lateral size: their edges become more compact; however, they still preserve a marked branched morphology. The zoomed image reported in Figure 2 for a 11 nm thick film reveals the compactness of the rubrene domains and the conservation of a layered morphology. Indeed, as shown by the cross-sectional profile, the surface of the islands is composed by terraces at different levels (7 for Figure 2) with equal spacing, and the island edge is defined by rounded features. Note that, despite the irregular shape of the islands, the terraces on top of them display polygonal steps, with straight edges preferentially oriented parallel to a direction close to tetracene[010]. This latter observation gives a weak indication of a preferential orientation of the film phase. In the background of Figure 2 (see also Supporting Information, Figure S2), one can also observe the presence of small grains. These are evidenced only when the tetracene surface undergoes the film deposition and are attributed to amorphous domains originating from the cocrystallization of rubrene and tetracene. This is possible because at 100 °C the desorption rate of tetracene is not negligible.12 A more reliable method for detecting the presence of rotational domains consists in collecting the torsional shear signal during AFM imaging;21 in our case, no contrast variation was registered (not reported), in accordance with uniaxially oriented islands. For a thorough investigation of the surface structure and epitaxial orientation of this system, we performed
Figure 2. AFM images (2 × 2 µm2) showing the details of the surface morphology of a crystalline domain of a 11 nm thick rubrene thin film deposited on tetracene(0 0 -1). The cross-sectional profile below the image, taken along the white line, puts in evidence the presence of monomolecular layers with spacing corresponding to d(200) ) 1.343 nm of the orthorhombic polymorph of rubrene. The crystallographic orientation of the substrate surface is indicated in the top left.
an atomic-scale analysis with AFM; the results are reported in Figure 3. Figure 3a shows the surface corrugation measured with AFM in a region of the sample exposing the bare substrate surface. A statistical analysis performed averaging over different images collected on the bare substrate gives a rhombic unit cell with parameters a ) 6.1 ( 0.1 Å, b ) 7.8 ( 0.2 Å, and γ ) 87.4° ( 0.6°, fully consistent with a (0 0 -1)-oriented crystal of tetracene (a ) 6.06 Å, b ) 7.84 Å, γ ) 85.79°, see also Supporting Information, Figure S3).22 In Figure 3b, the same procedure is applied on a terrace on the surface of a rubrene island; the corrugation reported there is the same as that measured over several tens of different islands. The image contrast reveals a rectangular unit cell with parameters b ) 7.1 ( 0.2 Å and c ) 14.6 ( 0.2 Å; these values agree with the orthorhombic (Cmca) form of rubrene, which for (100)-oriented domains exhibits surface cell parameters b ) 7.19 Å and c )
16180 J. Phys. Chem. C, Vol. 112, No. 42, 2008
Figure 3. AFM images (20 × 20 nm2) showing the atomic scale surface corrugation of the substrate (a) and of a crystalline domain of rubrene (b). The insets show the result of the Fourier synthesis of the image contrast by selecting enhanced intensity observed in the Fourier transforms reported in (c) (see also Supporting Information, Figure S3). (c) The superimposed Fourier transforms of image (a) and (b) are reported; reciprocal lattice points are highlighted in yellow for tetracene and in red for rubrene. White arrows indicate the coincidence of the reciprocal vectors tetracene(110) and rubrene(012). (d) A structural model of the tetracene/rubrene interface as deduced by the AFM analysis is reported.
14.43 Å.20 For determining the epitaxial relation of the substrate and overlayer, we performed the Fourier analysis of the atomicscale contrast shown in Figure 3a,b, and we reported the results in the same reciprocal space coordinate system (Figure 3c). The reciprocal surface lattices of the two materials are easily individuated and highlighted in different colors; in reciprocalspace, it is observed that the vectors rubrene(012) and tetracene(110) coincide. In real-space, this is equivalent to the coincidence of the rubrene[021] and tetracene[1 -1 0] crystallographic directions (see also Supporting Information, Figure S3). It follows that brubrene forms an angle of 80.0° with atetracene and an angle of 5.7° with btetracene. Coming back to Figure 2, in the light of these considerations the straight steps on the surface of rubrene islands are parallel to brubrene; this is in full agreement with the single crystal structure of rubrene, exhibiting closepacked layers not only within (200) planes but also within (002) ones.20 What is remarkable at this stage is that all islands display a unique orientation, with epitaxial relation rubrene(100)//tetracene(001) with rubrene[021]//tetracene[1 -1 0]. Before discussing the physical rationale for this orientation, it is worth
Letters drawing some considerations of the interface between tetracene(001) and rubrene(100). The (001) surface of tetracene is lined up with hydrogen atoms forming a rhombic unit cell with p1 plane symmetry. Due to the triclinic symmetry of tetracene, the (001) surface and the (0 0 -1) one are not equivalent, as they form a couple of enantiomers. This surface has been already employed for growing heteroepitaxial R-quaterthiophene nanostructures, giving rise to two phases of the overlayer with uniaxial orientations.12 For the R-quaterthiophene/ tetracene system, a fundamental role was found to be played by the main corrugation originated by hydrogen atom rows running along the [1 -1 0] direction of the (001) face of tetracene in orienting the overlayer. These rows give a corrugation with periodicity equivalent to the spacing among (110) planes d(110) ) 4.96 Å. On the other hand, the (100) surface of rubrene has a rectangular unit cell with pgg2 plane symmetry. If one tries to calculate geometrical coincidences between the surface lattice of tetracene and that of rubrene, the two structures result incommensurate (see Supporting Information, Figure S4). This means that the driving force for epitaxial ordering is not simple lattice match. If the information of incommensurism is joined to the experimental identification of the coincidence of the nonprimitive reciprocal lattice vectors tetracene(110) and rubrene(012), we can now identify this epitaxial relation as a lineon-line coincidence.10 The motivation of coincidence of these peculiar lines resides in the crystallo-chemical features of the rubrene(100) surface. This surface is lined up with hydrogen atoms of the lateral phenyl rings bonded to the tetracene core of the rubrene molecules. By an in-depth observation of the surface defined by the convolution of van der Waals radii of superficial atoms, one realizes that the main corrugations of rubrene(100) are originated by rows of hydrogen atoms running along the direction; the periodicity of such rows corresponds to the spacing among (012) planes d(012) ) 5.08 Å. Thus, despite the absence of lattice match, the contact surfaces of substrate and overlayer find a misfit of only 2.4% between their prominent corrugations when rubrene[021]// tetracene[1 -1 0]. The structural model sketched in Figure 3d shows the arrangement of molecules at the interface between a rubrene island and the substrate, putting in evidence the surface furrows running along rubrene[021] and tetracene[1 -1 0], as well as the accordance between the related corrugations. As a final remark, one should note that the same coincidence is in principle achieved for two rotational domains of rubrene, rotated by 90.18° with respect to each other (angle between [021] and [0 2 -1]). Nonetheless, these two configurations are no more symmetrically equivalent when the interface is formed, and then a preferred orientation is eventually expected. In our experimental statistical analysis of the film surface, we never found rotational domains with azimuthal angles of 90.18°. Then, we can conclude that a unique orientation is actually achieved. We have demonstrated the successful growth of highly oriented crystalline rubrene nanostructures accomplished by selecting substrates with an atomic-scale corrugation matching the prominent corrugation of the close-packed planes of the crystal structure of the overlayer. In our case, the natural corrugation of the (001) surface of tetracene, with 4.96 Å periodicity, allows sufficient adhesion to induce solidification of rubrene in its crystalline orthorhombic phase; furthermore, the low symmetry and chirality of the (001) surface of tetracene selects the nucleation of rubrene crystalline domains with a unique orientation. Even small lattice mismatches or small deviations of the corrugation periodicity from the value of the
Letters (100) surface of rubrene (5.08 Å) prevent the crystallization of the overlayer, which then solidifies in an amorphous phase. The heterostructure described in this work represents the first all-organic crystalline heterostructure obtained with rubrene; as such, it represents the starting point for the development of a thin film technology based on highly oriented rubrene nanostructures. The rubrene/tetracene heterostructure is potentially useful for an OFET configuration since rubrene assumes an inplane orientation parallel to the crystallographic direction of highest carrier mobility;3 moreover, while tetracene molecules are almost orthogonal to the interface plane, rubrene molecules arrange with their tetracene core parallel to it. In this configuration, the light is emitted by tetracene along a direction orthogonal to the interface,23 whereas the overlayer emits light along an in-plane direction.24 This peculiar geometry of optoelectronically active materials is of interest for envisaging lightemitting OFETs coupling light-emission anisotropy with wavelength tunability. Acknowledgment. This work was supported by Fondazione Cariplo (Grant 2007/5205). Luisa Raimondo and Adele Sassella are kindly acknowledged for their support for thin film preparation. Supporting Information Available: Experimental procedures, optical micrographs, AFM morphology, atomic-scale images, geometric coincidence analysis. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Muccini, M. Nat. Mater. 2006, 5, 605. (b) Service, R. F. Science 2005, 310, 1762. (2) Podzorov, V.; Menard, E.; Borissov, A.; Kiryukhin, V.; Rogers, J. A; M.; Gershenson, E. Phys. ReV. Lett. 2004, 93, 086602. (3) (a) Podzorov, V.; Pudalov, V. M.; Gershenson, M. E. Appl. Phys. Lett. 2003, 82, 1739. (b) Podzorov, V.; Sysoev, S. E.; Loginova, E.; Pudalov, V. M.; Gershenson, M. E. Appl. Phys. Lett. 2003, 83, 3504. (c) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 1644. (4) (a) Briseno, A. L.; Mannsfeld, S. C. B.; Ling, M. M.; Liu, S.; Tseng, R. J.; Reese, C.; Roberts, M. E.; Yang, Y.; Wudl, F.; Bao, Z. Nature 2006,
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16181 444, 913. (b) Mannsfeld, S. C. B.; Briseno, A. L.; Liu, S.; Reese, C.; Roberts, M. E.; Bao, Z. AdV. Funct. Mater. 2007, 17, 3545. (5) Ka¨fer, D.; Ruppel, L.; Witte, G.; Wo¨ll, Ch. Phys. ReV. Lett. 2005, 95, 166602. (6) Ka¨fer, D.; Witte, G. Phys. Chem. Chem. Phys. 2005, 7, 2850. (7) Hsu, C. H.; Deng, J.; Staddon, C. R.; Beton, P. H. Appl. Phys. Lett. 2007, 91, 193505. (8) Haemori, M.; Yamaguchi, J.; Yaginuma, S.; Itaka, K.; Koinuma, H. Jpn. J. Appl. Phys 2005, 44, 3740. (9) Zeng, X.; Wang, L.; Duan, L.; Qiu, Y. Cryst. Growth Des. 2008, 8, 1617. (10) Mannsfeld, S. C. B.; Leo, K.; Fritz, T. Phys. ReV. Lett. 2005, 94, 056104. (11) (a) Koller, G.; Berkebile, S.; Krenn, J. R.; Netzer, F. P.; Oehzelt, M.; Haber, T.; Resel, R.; Ramsey, M. G. Nano Lett. 2006, 6, 1207. (b) Oehzelt, M.; Koller, G.; Ivanco, J.; Berkebile, S.; Haber, T.; Resel, R.; Netzer, F. P.; Ramsey, M. G. AdV. Mater. 2006, 18, 2466. (c) Campione, M.; Sassella, A.; Moret, M.; Papagni, A.; Trabattoni, S.; Resel, R.; Lengyel, O.; Marcon, V.; Raos, G. J. Am. Chem. Soc. 2006, 128, 13378. (12) Campione, M.; Raimondo, L.; Sassella, A. J. Phys. Chem. C 2007, 111, 19009. (13) (a) de Boer, R. W. I.; Klapwijk, T. M.; Morpurgo, A. F. Appl. Phys. Lett. 2003, 83, 4345. (b) Butko, V. Y.; Chi, X.; Ramirez, A. P. Solid State Commun. 2003, 128, 431. (c) Kim, T. -H.; Lee, J. H.; Kim, J. -H.; Seoul, C. Mater. Res. Soc. Symp. Proc. 2006, 920, 39. (14) Tseng, R. J.; Chan, R.; Tung, V. C.; Yang, Y. AdV. Mater. 2008, 20, 435. (15) Gundlach, D. J.; Nichols, J. A.; Zhou, L.; Jackson, T. N. Appl. Phys. Lett. 2002, 80, 2925. (16) Hepp, A.; Heil, H.; Weise, W.; Ahles, M.; Schmechel, R.; von Seggern, H. Phys. ReV. Lett. 2003, 91, 157406. (17) Takahashi, T.; Takenobu, T.; Takeya, J.; Iwasa, Y. AdV. Funct. Mater. 2007, 17, 1623. (18) Chu, C.-W.; Shao, Y.; Shrotriya, V.; Yang, Y. Appl. Phys. Lett. 2005, 86, 243506. (19) Laudise, R. A.; Kloc, Ch.; Simpkins, P. G.; Siegrist, T. J. Cryst. Growth 1998, 187, 449. (20) Jurchescu, O. D.; Meetsma, A.; Palstra, T. T. M. Acta Crystallogr. 2006, B62, 330. (21) Puntambekar, K.; Dong, J.; Haugstad, G.; Frisbie, C. D. AdV. Funct. Mater. 2006, 16, 879. (22) Holmes, D.; Kumaraswamy, S.; Matzger, A. J.; Vollhardt, K. P. C. Chem.sEur. J. 1999, 5, 3399. (23) Tavazzi, S.; Raimondo, L.; Silvestri, L.; Spearman, P.; Camposeo, A.; Polo, M.; Pisignano, D. J. Chem. Phys. 2008, 128, 154709. (24) Tavazzi, S.; Borghesi, A.; Papagni, A.; Spearman, P.; Silvestri, L.; Yassar, A.; Camposeo, A.; Polo, M.; Pisignano, D. Phys. ReV. B 2007, 75, 245416.
JP806877E
